6 Answers
6

That is a very hard question to answer, because it depends on tier and what you mean by the numbers. However, here are my leading candidates.

Cleric. The cleric has more utility heals than any other class, and its heals are bigger. You can't overestimate the power of adding the Wisdom modifier to most of your heals, plus the cleric has an at-will that heals, which is unique to that class. When you want to bring someone from bloodied to full hit points in one go, you want the cleric.

Warlord. The warlord's heals aren't as big, but if you take the Fight On paragon tier feat you can pump out four Inspiring Words an encounter by level 16; this is good flexibility.

Bard. The bard's heals aren't particularly big either, but a valorous bard produces a ridiculous amount of temps -- particularly if she's a tiefling with the Stirring Song of Baator feat and a flaming weapon. And the great thing about temps is that they're not wasted if someone at full health receives them.

Shaman. You want to go Protector Spirit for best effect; your heals are, again, not huge but you can heal multiple people at once with flexibility no other class can match.

To summarize: clerics win for huge single-target healing. Warlords probably have the most stamina. Bards are right up there with warlords, albeit via temps rather than pure heals; and shamans are unmatched for party healing.

(And of course, there's nothing wrong with the ardent or the runepriest. Runepriests give really good buffs to their party, which is the other half of the leader's role; ardents have a superb range of at-wills and also grant temps rather easily. A well-played leader speeds up combat by buffing allies along with healing.)

Therefore, the following list will assess how much damage has been ameliorated over the course of an encounter, looking at every leader class, but only the "best" damage ameliorator of the class.

For all of these classes, we will assume that 2 level equivalent enemies are attacking the defender for the encounter, and that there will be one Opportunity Attack made by the enemies during the encounter. All math will be at level 11 for purposes of comparison.

Primary and secondary stats will be even, starting at base 18, at 21 (+5) Tertiary stat at +3.

Enemies hit 50% of the time, for 19 damage on average.

The combat will be 5 rounds long and there are 4 other party members, and everyone will hit 50% of the time.
A standard surge at level 11 will be 19.
I will assume 1 daily use, 3 encounters, and 1 at-will, with up to 2 utility if they are minor. and 1 Action pointed action.

The beauty of Artificer is in Resistive Formula + Armor of Shared Valor + Enhanced Resistive Formula + Potent restorables. = Person in the group with largest surge grants: +1 AC, as free action (read: when hit) free surge as THP + 2*con (to 2 people) + half to artificer. = 5*(19+10+4)=165 THP over the course of an encounter (at the net cost of 2 surges after the second encounter from people who've taken the least damage). + 4*(.05*19) blocked from the AC = 3.8 At the *start* of the encounter, so you don't have to deal with emergency after-the-fact healing.

Runepriest PP Rune Shield 175 (This number feels low, but the runepriest is all about defensive buffs, and so it's hard to quantify. If there's a better way to calculate "avoided" damage, please note it in comments. Still, this class is significantly better than this healing rating implies, it's also a PH3 class, so it hasn't received much love.)

Mark of warding is vital to the build. +1 to all granted defense buffs.

The best focused healer is the pacifist cleric, followed by the Inspiring Warlord (very counterintuitively, but those mass-heals boost healed damage incredibly.)

The best healer that is focused on other things is the Artificer, but he's almost as boring to play (having played one) as the pacifist healer, as he must use magic weapon as his at-will, because it's so good. Happily, interrupts make for an actually interesting fight. The Arti is easily the cheapest healer, dropping 145 THP at the start of the fight for 2 surges from the most healthy at the end. Ardent is a great "get in there and swing" and I suspect her numbers are a touch low.

There are some curious racial or non-class based PPs that modulate the above healing.

Rainbringer requires the use of dark sun themes. The use (not hitting) of any power with a healing keyword gives adjacent allies THP equal to half of their healing surge value.

There are a number of "spend an AP, grant a surge" PPs

the Sanctuary Guardian has an impressive level 12 utility, as well as: Ghallanda's Sanctuary (16th level): When any ally within 5 squares of you takes a total defense or second wind action, that ally gains temporary hit points equal to 5 + your Charisma modifier. -- While, nominally speaking, 10 or so THP isn't worth a standard, allies who regularly take total defense actions will appreciate this, as well as any kind of second-wind heavy group.

The shaman multiclass, while costing 2 feats, grants the character an encounter heal

Healing Paths (16th level): When any ally within 5 squares of you or your spirit companion spends a healing surge to regain hit points, he or she regains additional hit points equal to your Wisdom modifier.

As a consequence, the two primal healing classes add wis+con on top of anything else they're healing at level 16, and even non-primal classes can gain significant benefits.

Master Preserver has a fascinating level 16 Surging Vitality (16th level): Whenever you spend a healing surge, one ally within 10 squares of you regains hit points equal to twice your primary ability modifier. In addition, you gain a +1 bonus to attack rolls with arcane attack powers until the end of your next turn.

As any kind of defender role, this could make you an exemplary secondary healer.

The Compassionate Healer PP works well with any leader, especially ones that can grant themselves THP.

The Miracle Worker PP can be quite effective for the Druid Sentinel with the right feats.

Spiral Wind's Ally Druid PP is one of the better non-leader specific PP in the game:
While you are not in beast form, any bloodied ally within 5 squares of you who spends a healing surge regains additional hit points equal to your Wisdom modifier.

The Paladin's Hospitalier presents a remarkable amount of healing for violating his mark. With some kind of reliable marking scheme, this could enhance a leader's healing significantly.

The Warforged Lifeseeker presents a remarkable amount of self-healing for a leader, allowing them to play off-tank without needing to spend healing resources on their behalf, especially as there are a number of at-will powers that heal allies, they can theoretically gain HP every round for free.

And of course there are innumerable feat combinations. The best feat to take for parties that are always short of healing is the Tribal feat Enduring Mountain:

Benefit: Whenever you spend a healing surge, you regain 2 additional hit points. You regain 1 more additional hit point for each ally within 10 squares of you who has this feat, to a maximum of 5 additional hit points.

While everyone has to take this feat for it to be effective, the +7 HP granted by the feat outweigh (until epic) any healing benefit granted by the PPs above or in the class calculations.

Good analysis. A good formula to calculate damage mitigated through defenses is really needed. As a sample, a rune priest 11, Rune Shield Paragon Path can provide +4 to defenses of all adjacent allies: +2 with the 9th level Power "Word of Divine Providence" and +2 with the new Runic State of the Paragon Path. Also, may the Mark of Warding can apply as well, providing +6 to all defenses or ~30% less chance to be hit 1/day. At 14th level is possible to provide +8CA with rune of exchange or rune of binding at-will powers, which makes many tanks and off tanks nearly invulnerable though CA (39CA+).
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user4491Sep 27 '12 at 17:40

I don't have the numbers on hand to back me up but I would bet that a Pacificst Cleric outdoes everyone on the healing across every tier-- not just through direct healing, but also the effects of powers like Astral Seal.

I would think so too, but I'd like to see the numbers.
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C. Ross♦Sep 9 '10 at 17:01

2

Between the wis to heals, pacifist feat and the healing domain's interaction with astral seal, cleric wins on all levels at least as far as pure healing goes. It's not just that clerics have the best mechanics, its that there is so much content for clerics that they have more feats, powers and PP then any of the other leaders (except maybe warlord). In D&D, more options almost always equates to more power.
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SladeWestonSep 9 '10 at 18:46

1

Important note about Pacifist Clerics - while you do get Severely Awesome Healing, you get that by sacrificing nearly all your offensive capability. And Awesome Healing does run out if the party doesn't have enough oomph to kill things without you.
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Allen GouldFeb 21 '11 at 3:04

Astral seal almost always hits (+13
vs will is huge at 6 level) and heals
11 (or 16 if beacon of hope is
active) plus 3 temp. This is +5 if
Spirit of Healing is up.

Healing
words heal and insane HS+2d6+9 (+5 if
BoH)

You can use your CD to heal
bloodied allies HS+1d6+9 in a Burst 5

That's not to mention the temp hp, saving throws and status removal effects you have. Also worth noting is that none of these powers work against your pacifist feat. Be warned, this character was about as fun to play as watching paint dry and was very disrupting at the table.

Bards are severely underwhelming when it comes to healing. They're far more proactive enablers than their reactive cleric cousins. The idea of "lotsa heals" is better exemplified by one power that can heal multiple people, and there are tons of those. Again, bards don't really focus in that regard, and sacrificing feats for daily heals... is trying to make a bard into something she's not.
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Brian Ballsun-Stanton♦Feb 19 '11 at 1:39